Developing apparatus

ABSTRACT

A developing apparatus having a resilient developing roller for developing an image with a single-component toner includes a contacting mechanism for moving the developing roller into contact with a surface of an image carrier for developing the image and moving the developing roller in a forward direction at substantially the same speed as the image carrier. The width of a developing nip and a depth of bite between the image carrier and developing roller is set such that the traveling speed of the local portion of the developing roller that contacts the image carrier gradually decreases from a point at which contact starts and then gradually returns to the original speed. The width of the developing nip is at least 4 mm, the hardness of the developing roller is 20-40 degrees, the diameter of the developing roller is 40-100 mm, and the bite depth is equal to or greater than {fraction (1/100)} the radius of the developing roller. The developing apparatus can also include a layer thickness regulating roller has end portions that prevent adhesion of toner thereto, the contacting mechanism includes gears on the image carrier and developing roller that are slightly meshed even when the developing roller is out of contact with the image carrier, and the image size being at least an A 2  size.

TECHNICAL FIELD

This invention relates to a developing apparatus used in anelectrophotographic apparatus and, more particularly, to a developingapparatus in which an electrostatic latent image formed on an imagecarrier is developed and visualized by a non-magnetic single-componentdeveloper.

BACKGROUND ART

Generally, in an electrophotographic apparatus such as a copier, printeror plotter that utilizes electrophotography, the electrostatic latentimage of a desired image is formed on an image carrier such as aphotosensitive drum and a developer is supplied by a developingapparatus to develop the electrostatic latent image so that a visibletoner image is formed on the image carrier. A two-component developercomprising toner and carrier, and a magnetic single-component developeror non-magnetic single-component developer comprised of toner alone areknown as developers. Various developing systems suited to thesedevelopers have been developed and proposed.

Non-magnetic single-component developers in particular have variousadvantages but utilization thereof in actual machines has been slow. Inrecent years, however, utilization in actual machines has spread rapidlywith the development of new or improved developers, which are the resultof performance enhancement, such as polymer toners that excel in imagereproducibility and transfer.

A contact-type developing apparatus has been proposed as a developingapparatus that uses a non-magnetic single-component developer, in whicha flexible developing roller exhibiting electroconductivity or anappropriate electrical resistance is used as a developer carrier forsupplying a developer to an image carrier, a thin layer of the developeris formed on the surface of the roller and then the roller is broughtinto contact with the surface of the image carrier at a suitablepressure to develop the image. It is known that such a contact-typedeveloping apparatus can be used preferably in development which doesnot require an edge enhancement effect and in which it is required thatthe developing characteristics of line drawings and pictorial images beidentical, as in a digital printer in which an image is formed bymonochrome bi-level values. This is known also as a cleanerless systembecause cleaning can be carried out at the same time as development.

In an early apparatus of this kind, the arrangement is such that aphysical or mechanical load brought about by contact between thedeveloping roller and the surface of the image carrier is mitigated bymaking the peripheral speed of the elastic developing roller, whichrotates in the forward direction, and the peripheral speed of the imagecarrier approximately identical. However, difficulties arise in terms ofimage quality relating to image definition, texture smudging andfogging. An arrangement which provides a difference in speed between theperipheral speed of the image carrier and the peripheral speed of thedeveloping roller has been proposed as an improvement (e.g., see thespecifications of Japanese Patent Nos. 2598131 and 2803822).

In accordance with the proposed apparatus, the surface of the developingroller is brought into sufficient sliding frictional contact with thesurface of the image carrier via a toner layer owing to the differencein the peripheral speeds between the developing roller and imagecarrier, whereby excellent development and cleaning are carried outsimultaneously. In order to achieve such sliding frictional contact, thedeveloping roller is set to as to rotate at a peripheral speed that is1.5 to 4 times that of the image carrier. Further, it is disclosed thatthe contact width between the developing roller and surface of the imagecarrier, namely the development nip zone, should be equal to or morethan 50 times but equal to or less than 500 times the volume averageparticle diameter of the developer particles.

In experiments, however, the Inventors have found that several problemsstill need to be solved in terms of structure and requirements in orderto obtain fully satisfactory image quality, especially the fact thatsome points that do not give rise to problems in small type printersthat develop small-size images do represent major problems whendeveloping large-size images such as images of size A2, A1 and A0 bylarge type printers.

One problem is as follows: When the force with which the developingroller comes into pressured contact with the image carrier iscomparatively large and the peripheral speed of the developing rollerdiffers from that of the image carrier, the toner on the surface of thedeveloping roller is pulverized by the pressure of sliding contact,resulting in rapid toner deterioration. Further, toner adheres to (orbecomes fused to) the surface of a developer-layer regulating member,which regulates the thickness of the layer of developer that is formedon the developing roller, owing to the development of, say, severalthousand meters, and the adherence of the toner prevents the formationof a uniform thin layer of the developer, thereby causing white stripesto appear on the image. An additional drawback is that the image carrierrotates unevenly owing to the action of pressing force applied to theimage carrier by the developing roller rotating at a differentperipheral speed. Furthermore, in a large-size electrophotographicapparatus for developing large-size images, the torque for driving thedeveloping roller is fairly large in order to produce the aforementionedsliding contact. This is uneconomical.

Further, in the prior art described above, maintaining the width of thedevelopment nip zone (“nip width”) is a major factor in achieving gooddevelopment and the nip width is to be made 50 to 500 times the averageparticle diameter of the toner. Accordingly, if the diameter of thetoner used in such development is on the order of 8 μm, the nip widthwill be 0.4 to 4 mm, which is 50 to 500 times this diameter. In a casewhere a developing roller having a diameter of 40 mm is made to contactan image carrier having a diameter of 120 mm, for example, thepositional dimensions between the developing roller and image carriermust be maintained in such a manner that the depth of bite of thedeveloping roller into the image carrier will be 0.001 to 0.134 mm.Considerable dimensional precision and setting of position will berequired of these members.

Even if this is a soluble problem in a small-size developing apparatusof size A4 or A3 having an image carrier or developing roller ofcomparatively small length, it is a problem of considerable difficultyin a large-size developing apparatus having a developing roller of largelength. For example, finishing of the developing roller usually isperformed by grinding. In an instance where an A0-size image is to bedeveloped, a roller having a length of about 850 mm must be machined asthe developing roller. Finishing the roller to a diametric error of tensof microns over its entire length so as to satisfy the above requirementis considerably difficult and results in costly machining. Further, in acase where the amount of wobble of an A0-size image carrier at rotationthereof and the amount of wobble of the developing roller are each 0.1mm and, hence, there is an error in the diameter between these members,the depth of bite of the developing roller into the surface of the imagecarrier varies from area to area and, as a result, image density varieslocally and gives rise to uneven development.

In addition, a developing roller made of a resilient material such asrubber exhibits a large coefficient of thermal expansion and thereforethe diameter thereof tends to change with a change in ambienttemperature. As a result, a problem which arises is that the nip widthbetween the image carrier and developing roller varies with a change intemperature. This is a further cause of uneven development.

Thus, in the prior art as described above, satisfactory mechanicalprecision for coping with the environment of use is difficult to obtainin cases where a large-size image is developed. As a result, stable,uniform images cannot be obtained consistently.

Furthermore, leakage of toner from both ends of the developing roller tothe exterior of the developing apparatus is one problem with acontact-type developing apparatus that uses non-magneticsingle-component toner. That is, because of the non-magnetic nature ofthe toner, the toner cannot be gathered together by magnetic force as inthe manner of the conventional magnetic-developer system. Severalalternative proposals for preventing such leakage have been made.

Most of these proposals place a lubricating seal between both ends ofthe developing roller and the side plates of the developing apparatus,thereby attempting to prevent leakage of the toner. However, in anarrangement in which such seals are placed, the seals wear out ordeteriorate owing to long-term use and a satisfactory sealing effectcannot be maintained.

DISCLOSURE OF THE INVENTION

The present invention has been devised in view of the above-mentionedcircumstances and seeks to provide a developing apparatus in whichexcellent development is possible at all times even when developinglarge-size images of size A0 and A1. A further object of the presentinvention is to provide a developing apparatus in which amount of biteof a resilient developing roller into an image carrier and width of adevelopment nip zone can be made suitable values for the sake ofachieving the excellent development mentioned above.

A further object of the present invention is to provide a developingapparatus in which it is possible to prevent toner leakage through asimple arrangement by utilizing a layer-thickness regulating roller forforming a thin layer on a developing roller.

According to the present invention, the foregoing objects are attainedby providing a developing apparatus for forming a thin layer of toner,which comprises a non-magnetic single-component toner, on a resilientdeveloping roller, bringing the roller into abutting contact with thesurface of an image carrier, whereby toner on the resilient developingroller is supplied to an electrostatic latent image that has been formedon the surface of the image carrier, thereby developing theelectrostatic latent image, and moving the image carrier and theresilient developing roller in a forward direction in such a manner thattraveling speed of the image carrier and peripheral speed of theresilient developing roller become substantially identical, whereinwidth of a development nip zone that extends from a point at which theresilient developing roller starts to contact the image carrier to apoint at which the resilient developing roller breaks contact with theimage carrier is equal to or greater than 4 mm, preferably 5 to 10 mm,rubber hardness of the resilient roller is 20 to 40° and diameter of theresilient developing roller falls within the range 40 to 100 mm, andamount of bite by which the surface of the image carrier bites into theresilient developing roller is set so as to be equal to or greater than{fraction (1/100)} of the radius of the resilient developing roller,characterized in that the amount of bite and the width of thedevelopment nip zone are set in such a manner that the resilientdeveloping roller breaks contact with the image carrier while travelingspeed of a local portion of the resilient developing roller in contactwith the surface of the image carrier gradually decreases from the pointat which contact starts and thenceforth gradually returns to theoriginal speed owing to resilience of the resilient developing rollerper se.

Further, the amount of bite of the resilient developing roller into thesurface of the image carrier is set so as to be equal to or greater than{fraction (1/40)} of the radius of the developing roller.

Further, the depth of bite of the resilient developing roller into thesurface of the image carrier is set to be equal to or greater than{fraction (1/100)} of the radius of the developing roller. Inparticular, the depth of bite is set to 0.2 to 3 mm. Further, it ispreferred that the thin layer of toner formed on the resilientdeveloping roller be a uniform layer of one to three layers of toner.

Furthermore, rubber hardness of the resilient roller is 20 to 40°, asmeasured according to the JIS K 6253 (Type A) standard.

Means for forming the thin toner layer on the resilient developingroller comprises a layer-thickness regulating roller placed inopposition to the developing roller.

The layer-thickness regulating roller has a central portion along theaxial direction thereof and end portions that are electrically insulatedfrom the central portion, and a bias voltage for preventing adhesion oftoner to the end portions of the resilient developing roller is appliedto the end portions.

Further, the apparatus is equipped with a scraping blade provided inpressured contact with a central portion of the layer-thicknessregulating roller with respect to the axial direction thereof and withend portions of the roller, the scraping blade being so adapted as toscrape off toner that has adhered to the layer-thickness regulatingroller.

Further, the developing roller is provided so as to be capable ofcontacting and separating from the surface of the image carrier in orderto assure a suitable positional relationship between the developingroller and image carrier, both ends of the developing roller areprovided with contact rollers, and the contact rollers are brought intoabutting contact with both ends of the image carrier to regulate thewidth of the development nip zone.

Furthermore, the image carrier and developing roller are moved so as tomesh a gear provided on a flange of the image carrier with a gearprovided on an end of the developing roller, whereby drive from theimage carrier is transmitted to the developing apparatus.

In particular, the developing apparatus is characterized by furtherhaving separation means for causing the developing roller to separatefrom the image carrier, wherein the gear provided on the image carrierand the gear provided on the developing roller are made to mesh slightlywhen the image carrier and the developing roller are in a separatedstate.

Further, the present invention is characterized by having a cam inabutting contact with a portion of the developing apparatus for beingturned at introduction of power to thereby move the developing apparatusin such a manner that the developing roller is pressed against the imagecarrier, and by provision of a capacitor charged when power is beingintroduced, wherein the capacitor is switched over to act as a powersource at cut-off of power, thereby rotating the cam and moving thedeveloping apparatus in such a manner that the developing roller movesin a direction in which it separates from the image carrier.

Furthermore, in accordance with the present invention, there is provideda developing apparatus for forming a thin layer of toner, whichcomprises a non-magnetic single-component toner, on a resilientdeveloping roller, and bringing the roller into abutting contact withthe surface of a drum-shaped image carrier, whereby toner on theresilient developing roller is supplied to an electrostatic latent imagethat has been formed on the surface of the image carrier, therebydeveloping the electrostatic latent image, characterized by comprising:means for rotating the image carrier and the resilient developing rollerin a forward direction in such a manner that peripheral speed of theimage carrier and peripheral speed of the resilient developing rollerbecome substantially identical; and means for regulating depth of biteof the resilient developing roller into the surface of the image carrierand width of a development nip zone that extends from a point at whichthe resilient developing roller starts to contact the image carrier to apoint at which the resilient developing roller breaks contact with theimage carrier.

Furthermore, the invention is characterized in that the moving meanscomprises mutually meshing gears formed on ends of respective ones ofthe image carrier and developing roller, and in that the regulatingmeans comprises rollers provided on both ends of the developing rollerand the peripheral surfaces of which are brought into abutting contactwith both ends of the image carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view illustrating a preferred embodiment of adeveloping apparatus according to the present invention;

FIG. 2 is a schematic top view of the developing apparatus according tothe present invention;

FIG. 3 is a schematic side view illustrating a state in which thedeveloping apparatus has been moved in a direction in which it separatesfrom an image carrier;

FIG. 4 is a perspective view of a developing roller and layer-thicknessregulating roller;

FIG. 5 is a sectional view of the layer-thickness regulating roller; and

FIG. 6 is an enlarged sectional view of the image carrier and developingroller.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be describedwith reference to the drawings.

FIG. 1 is a schematic side view of a developing apparatus to which thepresent invention is applied, and FIG. 2 is a schematic top view. In theFigures, numeral 1 denotes an image carrier moved in the directionindicated by arrow A, namely in the clockwise direction. By way ofexample, the image carrier comprises an arrangement in which the surfaceof a drum-shaped substrate made of aluminum or the like is equipped withan electrophotographic photosensitive drum. A well-known photosensitivedrum such as an OPC (Organic Photo-Conductor) photosensitive drum oramorphous silicon photosensitive drum can be used as the image carrier1. It should be noted that the image carrier 1 may have a shape otherthan that of a drum, namely the shape of a belt.

Though not shown (with the exception solely of the developingapparatus), the following are disposed about the periphery of the imagecarrier 1 along the direction A of rotation: an eraser lamp for erasingresidual electric charge from the image carrier 1; a charging device forcharging the image carrier 1 uniformly to a specific polarity; anexposure device such as an LED head for forming an electrostatic latentimage on the image carrier 1 by causing digital light information toimpinge upon the surface of the electrically charged image carrier 1; adeveloping apparatus 2 (FIG. 1) for reversal-developing theelectrostatic latent image by supplying toner to the image carrier 1,i.e., for forming a toner image by causing toner to affix itself to theexposed portion; a post-lamp for assisting in improving the tonertransfer efficiency by uniformly de-electrifying the image carrier 1 andtoner by uniformly exposing the surface of the image carrier 1 carryingthe toner image; and a transfer device for transferring the toner imageon the image carrier 1 to a transfer material such as paper. Theseunillustrated components surrounding the image carrier 1 can be selectedfrom among well-known devices with the exception of the illustrateddeveloping apparatus 2.

In this arrangement, the developing apparatus 2 has a developing vessel3 accommodating a single-component developer (referred to as “anon-magnetic single-component toner” or simply “toner” below) comprisingan insulating non-magnetic toner; a developing roller 4 comprising aresilient body; a layer-thickness regulating roller 5, which is placedso as to contact the developing roller 4 at a suitable pressure, forregulating the layer thickness of the toner formed on the developingroller 4 (owing to the roller shape in the illustrated example, thisdevice will be described below as the layer-thickness regulating roller5, although an alternate shape such as that of a plate may be used); asupply roller 6, which is provided in contact with the developing roller4, for supplying the developing roller 4 with toner; and a stirringmember 7 disposed in back of the supply roller 6. As will be set forthlater, the developing roller 4, supply roller 6 and layer-thicknessregulating roller 5 are connected to suitable bias power supplies sothat each is supplied with a prescribed bias voltage. It should be notedthat since the toner undergoes reversal development, use is made of atoner having a polarity identical with that of to which the imagecarrier 1 is charged.

These components will now be described in detail. A prescribed amount ofthe non-magnetic single-component toner is accommodated inside thedeveloping vessel 3. Disposed at a position opposing the image carrier 1is the developing roller 4, which has a length approximately equal tothat of the image carrier 1 and extends in a direction parallel to theaxis of the image carrier 1, in such a manner that part of theperipheral surface thereof is exposed to the side of the image carrier 1through an opening 8 formed in the developing vessel 3. The amount oftoner contained in the developing vessel 3 is such that the top of thesupply roller 6 will be exposed and is monitored constantly by a sensorprovided on a rear wall of the developing vessel 3. When the amount oftoner falls below a predetermined amount, the sensor issues a commandsignal so that the toner will be replenished from a toner cartridge 9.

The developing roller 4 includes a resilient intermediate layer 12formed about a center shaft 11 comprising an electrically conductiverigid body made of stainless steel or the like, and a resilient surfacelayer 13 formed on the outer periphery of the intermediate layer 12. Thedeveloping roller 4 is disposed in pressured contact with the surface ofthe image carrier 1, in such a manner that the surface layer 13 andintermediate layer 12 are resiliently deformed, over a nip zone width of4 mm or greater, preferably 5 to 10 mm, and rotates in the forwarddirection with respect to rotation of the image carrier 1, i.e., in acounter-clockwise direction B.

The center shaft 11 of the developing roller 4 is connected to a biaspower supply 14 a via a switch 31. The bias power supply 14 a applies abias voltage for preventing fogging of toner on the image background.The bias voltage is set to a value lower than the surface potential ofthe image carrier 1 by 100 to 500 V, preferably 300 to 400 V, in termsof absolute value. If the potential difference is made less than 300 V,the density of development will decline; if it is made 400 V or greater,cleaning will become difficult and it will tend to be difficult toobtain a high-quality reproduced image.

In the illustrated example, a second bias power supply 14 b the polarityof which is opposite that of the bias power supply 14 a is provided.Connection to either of these bias voltages is made by changing over theswitch 31. The changeover is made at a predetermined timing at the timeof image formation, which is when the developing roller 4 mainly takespart in development, and at such time that the developing roller 4 takespart mainly in cleaning before image formation or during idling betweenimage formation processes. That is, at the time of development, theswitch is changed over to a polarity identical with that of thepotential to which the surface of the image carrier 1 is charged; at thetime of cleaning, the switch is changed over to a polarity opposite thatof the potential to which the surface of the image carrier 1 is charged.

With reference again to the structure of the developing roller 4, theintermediate layer 12 formed on the outer side of the center shaft 11and the surface layer 13 formed on the outer peripheral surface of thisintermediate layer are formed by a resilient body of two layers ofdifferent properties having volume resistance values such that theintermediate layer 12 will have a comparatively high resistance and thesurface layer 13 a low resistance relative thereto, by way of example.For instance, silicone rubber can be used as one example of theseresilient bodies. In such case, it is preferred that the silicone rubberconstructing the intermediate layer 12 have a volume resistance of 10⁴to 10⁹ Ω·cm and that the silicone rubber constructing the surface layer13 have a volume resistance of 10⁵ to 10⁹ Ω·cm.

The developing roller 4 is not limited to such a two-layer structure; itmay have a single-layer structure or, conversely, a structure of threeor more layers. A material other than silicone rubber may be used. Aresilient material (inclusive of a porous foamed body) such as NBRrubber (acrylonitrile-butadiene copolymer rubber) or urethane rubber maybe used as the intermediate layer 12, and an article formed from aresilient body such as urethane rubber may be used as the surface layer13. Further, a single-layer-type developing roller, consisting of alayer of NBR rubber, urethane rubber or silicone rubber, etc., may beused.

The specific resistance of the resilient layer of a single-layerdeveloping roller or the specific resistance of the resilient layerscomprising the intermediate layer and surface layer of a multiple-layerdeveloping roller preferably falls within the range 10⁵ to 10⁸ Ω·cm, andthe rubber hardness preferably falls within the range 20 to 40°. Inparticular, the surface layer 13 (the outer surface in case of a singlelayer) preferably is formed by a material which has fine surfaceroughness in order to provide a toner transport capability, exhibits agood release property with respect to the toner and is separated fromthe toner in the triboelectric series.

The surface of the developing roller 4 differs depending upon theparticle diameter of the toner. If the average particle diameter of thetoner is 8 to 10 μm, then it is preferred that the surface of thedeveloping roller 4 have roughness of about 10 Ω·m. A material having alow hardness of about 10 to 20° used as the core, and a resilient bodyhaving a hardness of 20 to 40° is used as the surface layer. Such adeveloping roller 4 is disposed in such a manner that the depth of biteinto the image carrier 1 will be 0.2 to 3 mm, and preferably {fraction(1/100)} times, especially {fraction (1/40)} times, the radius of thedeveloping roller 4 in a case where the radius of the developing roller4 is less than 30 mm. The developing roller 4 carries out development byrotating at a peripheral speed substantially the same as that of theimage carrier 1. What is important here is the roughness of the surfacelayer 13 and the apparent hardness of the surface of the developingroller 4. A low apparent hardness for the surface of the developingroller 4 is preferred because the rotational torque will be smaller.

The supply roller 6 situated in back of the developing roller 4 isdisposed so as to extend in parallel with the axis of the developingroller 4 and contacts the developing roller 4 substantially along itsfull length. By way of example, the supply roller 6 comprises a foamedbody of urethane rubber mixed with finely divided carbon powder. Whilecontacting the developing roller 4 under a predetermined pressure, thesupply roller 6 rotates in a direction opposite that in which thedeveloping roller 4 rotates, i.e., counter-clockwise, supplies thedeveloping roller 4 with toner from within the developing vessel 3 andelectrically charges the toner on the developing roller 4 bytriboelectrification. The charging of the toner by the friction betweenthe developing roller 4 and supply roller 6 has a major influence uponquality of development. If charging is insufficient, fogging and densityunevenness will occur.

A center shaft 16 of the supply roller 6 is connected to a bias powersupply 14 c via a Zener diode (not shown) so that the shaft is suppliedwith a predetermined bias voltage. The bias position applied to thesupply roller 6 is set to a potential higher than the bias potential ofthe developing roller 4 by 100 to 200 V in terms of absolute value.Toner is transferred from the supply roller 6 to the developing roller 4by this potential difference.

The stirring member 7 provided in back of the supply roller 6 has acenter shaft 18 extending in the same direction as the axis of thedeveloping roller 4, and a stirring blade 19 provided on the shaft at aplurality of locations along the direction of the axis. The stirringmember 7 stirs the toner inside the developing vessel 3 by rotating andtransports the toner to the supply roller 6 so that the toner issupplied. In this example, the stirring blade 19 rotates in theclockwise direction.

The toner supplied in layer form to the developing roller 4 by thesupply roller 6 has its layer thickness regulated by the layer-thicknessregulating roller 5. The layer-thickness regulating roller 5 comprises aconductive or semiconductive roller body which, in terms of thedirection of rotation of developing roller 4, is disposed at a positionupstream of the area of contact between the developing roller 4 andimage carrier 1, i.e., upstream of the development nip zone, and has alength approximately the same as that of the developing roller 4. Thelayer-thickness regulating roller 5 is provided so as to rotate whilepart of its circumferential surface contacts the surface of thedeveloping roller 4 at a predetermined pressure. In the illustratedexample, the layer-thickness regulating roller 5 is placed directlyabove the developing roller 4. A bias voltage having a polarityidentical with that of the supply roller 6 is applied to thelayer-thickness regulating roller 5.

The layer-thickness regulating roller 5, which rotates in a directionopposite the rotating direction B of the developing roller 4, i.e.,counter-clockwise, acts in such a manner that some of the toner affixedto the developing roller 4 is allowed to remain in a thin layer (one tothree layers of the toner) on the developing roller 4 while the rest ofthe toner is removed by causing it to transfer to and be adsorbed by thecircumferential surface of the layer-thickness regulating roller. Theexcess toner thus transferred by being peeled off by the layer-thicknessregulating roller 5 is removed from the layer-thickness regulatingroller 5 by a resilient removal blade 21 disposed in such a manner thatits distal end is in abutting contact with the circumferential surfaceof the layer-thickness regulating roller 5.

The operation of the developing apparatus according to the presentinvention will be described next. In FIG. 1, first, with the imagecarrier 1 rotating in the direction of arrow A, the residual potentialon the image carrier 1 is removed by the eraser lamp, then the surfaceof the image carrier 1 is charged uniformly by a charging device such asa corona charging device or charging roller. Digital exposure is thencarried out by an exposure device to form an electrostatic latent imageon the image carrier 1. The latent image is transported by rotation ofthe image carrier 1 to the position where the where the image carriercontacts the developing roller 4 of the developing apparatus 2, namelyto the development nip zone.

Meanwhile, in the developing apparatus 2, the developing roller 4,supply roller 6 and stirring member 7 are rotated by driving sources(not shown) in the directions indicated by the respective arrowssubstantially at the same time that the image carrier 1 rotates, andpredetermined bias voltages are applied to respective ones of thedeveloping roller 4, supply roller 6 and layer-thickness regulatingroller 5.

Owing to rotation of the stirring member 7 and supply roller 6, thetoner inside the developing vessel 3 is stirred and supplied so that atoner layer is formed on the developing roller 4. The toner layer isregulated by the layer-thickness regulating roller 5 so as to become auniform thin layer of one to three layers, after which the toner iscarried to the development nip zone with rotation of the developingroller 4.

The toner of reduced thickness is supplied to the electrostatic latentimage of the image carrier 1 in the development nip zone and the imageis developed by attraction and affixing of the toner, whereby a visibletoner image is formed.

The toner image is transported to a transfer area (not shown) byrotation of the image carrier 1. Here, by virtue of the action of atransfer device such as a transfer corona or transfer roller, anelectric field whose polarity is opposite that of the toner is appliedfrom the back side of the transfer medium so that the toner istransferred onto the transfer medium. The toner image that has beentransferred is fixed onto the transfer medium at a fixing area, notshown. Meanwhile, residual toner on the image carrier not transferred tothe transfer medium is subjected to full-surface exposure by the eraserlamp so that it attains a potential approximately the same as thepotential of the dark portions of the image carrier (the potential ofthe image background). Next, charging and exposure for the formation ofthe next image are applied in a manner similar to that described aboveand the toner is then carried to the developing roller 4. The developingroller 4 allows the toner remaining on the image carrier 1 to berecovered in the developing apparatus 2 and supplies new toner to thesurface of the image carrier 1 so that the next electrostatic latentimage is developed.

Described next will be the drive sections of the developing roller 4 andthe like as well as the positional relationship between the developingroller 4 and the image carrier 1 according to the present invention.

FIG. 2 is a diagram useful in describing the drive section of thedeveloping apparatus 2. Flanges 10 are provided on both sides of theimage carrier 1. Each flange 10 is concentric with respect to the imagecarrier 1 and the outer circumferential surface thereof substantiallyagrees with the outer circumferential surface of the image carrier 1.One of the flanges 10 is provided with a gear 15 a. The gear 15 a isdisposed so as to mesh with a gear 15 b supported on one end of thecenter shaft 11 of developing roller 4. Contact rollers 17, theoperation of which will be described later, are provided on both ends ofthe center shaft 11 of developing roller 4. Each contact roller 17 ismade of resin or metal and has a disk-shaped configuration and isprovided on the center shaft 11 of developing roller 4 so as to becapable of rotating freely. The contact rollers regulate the amount ofpressure the developing roller 4 applies to the image carrier 1 (thedepth of bite) and rotate while in abutting contact with the ends of theimage carrier 1 or with the flanges 10. When the contact rollers 17 arethus abutted against the flanges 10, the developing roller 4 abutsagainst the image carrier 1 at the width of the prescribed width zoneand/or the depth of bite.

In a case where it is necessary to move the developing roller 4(developing apparatus) in order to prevent damage to the image carrier 1such as denting when the apparatus is not being used, the developingapparatus 2 is pulled away from the image carrier 1 (or loosened) by acam, described later. Alternatively, when the apparatus is operating,the contact rollers 17 are urged toward the image carrier 1 by the camuntil they come into abutting contact with the image carrier 1 orflanges 10.

With reference again to FIG. 1, numeral 20 denotes the cam, whichfunctions to bring the developing apparatus 2 into and out of contactwith the image carrier 1. The cam 20 is situated with its cam face inabutting contact with part of the developing apparatus 2. When power isintroduced from a power supply 28, the cam is rotated through aprescribed angle in accordance with a command from motor control means27 so that the developing apparatus 2 is moved in a direction that urgesit toward the image carrier 1, whereby the circumferential surface ofthe developing roller 4 is pressed against the circumferential surfaceof the image carrier 1. A capacitor 25 is provided so as to be chargedduring introduction of power in order that the cam 20 may be restored.When power is cut off, a changeover is made by switching means 30 insuch a manner that the capacitor 25 serves as a temporary power source.As a result, the cam 20 is rotated and causes the developing apparatus 2to move in a direction away from the image carrier 1 (FIG. 3). Numeral22 denotes a frame supporting the center shaft 11 of the developingroller 4 and the center shaft 16 of the supply roller 6.

Operation attendant upon operation of the cam 20 will now be describedin greater detail. FIGS. 1 and 3 are diagrams illustrating therelationship between the developing apparatus 4 and the image carrier 1when power is respectively introduced to and cut off from the apparatus.In FIG. 1, the switching means 30 has two switches. During introductionof power, one switch is connected to the power supply 28 and the otherswitch is connected to the motor control means 27. In a case where thispower supply is cut off in FIG. 3, the one switch is connected to thecapacitor and the other is connected to ground.

When a main switch (not shown) on the main body of the apparatus isoperated to turn on the power supply 28, the two switches of theswitching means 30 are changed over from the positions shown in FIG. 3to the positions shown in FIG. 1, whereby a motor 26 is connected to thepower supply. On/off control of the motor 26 is performed based upon asignal from the motor control means 27. When power is introduced fromthe power supply 28, the motor 26 is driven for a prescribed period oftime in accordance with an ON signal from the motor control means 27,i.e., until an angular position at which the cam face of the cam 20contacts part of the developing apparatus 2 shifts from theminimum-diameter position of cam 20 shown in FIG. 3 to the vicinity ofthe maximum-diameter position shown in FIG. 1. More specifically, byvirtue of such drive, the cam 20 turns in the direction of arrow c sothat the developing apparatus 2 is moved in the direction of the imagecarrier 1 gradually in accordance with the shape of the cam face tothereby press the developing roller 4 against the image carrier 1 insuch a manner that the developing roller 4 contacts the image carrierover the prescribed contact width (the width of the nip zone).

The turning of the cam (the rotational position of the cam) from theposition shown in FIG. 3 to the position shown in FIG. 1 is sensed byposition sensing means (not shown) such as a photosensor. The detectiondata is sent to the motor control means 27, whereby the motor controlmeans 27 controls the drive timing of the motor 26.

Further, when power supply 27 is turned on, a charging signal is outputfrom charging control means 29. The capacitor 25 is charged inaccordance with this signal.

Next, as shown in FIG. 3, when the power supply 28 is cut off, theelectricity that accumulated in the capacitor 25 by changeover of theswitching means 30 drives the motor 26 to turn the cam 20 through theprescribed angle in the direction of arrow c, whereby the position shownin FIG. 3 is attained. Owing to this rotation of the cam 20, thedeveloping apparatus 2 moves in a direction away from the image carrier1 instantaneously through the cam step, whereby the pressing force ofthe developing roller 4 is relaxed or removed.

When the developing apparatus 2 is moved toward the image carrier 1 bythe above operation of the cam 20 so that the developing roller 4 ispressed against the image carrier 1 over the prescribed width of the nipzone and/or by the prescribed depth of bite, the gear 15 a provided onthe image carrier 1 and the gear 15 b provided on the developing roller4 are meshed so that drive can be transferred. When the developingapparatus 2 is moved in the direction away from the image carrier 1 torelax or remove the pressing force of the developing roller 4, the gear15 a of the image carrier 1 and the gear 15 b of the developing roller 4are not completely separated from each other and the tips of the gearsare in slight mesh with each other. As a result, the gears can be meshedby the pressing force of the developing roller 4 without the tips of thegears 15 a, 15 b clashing with each other.

As shown in FIGS. 4 and 5, the layer-thickness regulating roller 5 is soconstructed that a central portion 23 a along the axial direction andend portions 23 b are electrically insulated from each other. In theillustrated example, a collar 23 c comprising an insulating resin issecured to a support shaft 23 d (shown only in FIG. 5) of the centralportion 23 a so as to insulate and cover the support shaft 23 d of thecentral portion 23 a and the end face of the central portion 23 a. Theend portion 23 b is secured to the outer side of the collar 23 c in sucha manner that its circumferential surface is flush with the surface ofthe central portion 23 a. The insulating method naturally is not limitedto one that relies upon the collar 23 c; another insulating method suchas one that uses an insulating coating may be used. Further, though onlyone end of the layer-thickness regulating roller 5 is illustrated inFIGS. 4 and 5, the other end also has a similar structure. Numeral 24denotes a gear for transmitting a driving force, which is from a drivingsource that is not shown, to the layer-thickness regulating roller 5.

Bias potentials that differ from each other are applied to theelectrically isolated central portion 23 a and end portions 23 b of thelayer-thickness regulating roller 5 (one of these potentials may beground potential, depending upon the potential of the developing roller4). Basically, the potentials are decided in such a manner that all ofthe toner on the end portions 4 b of the developing roller 4 will betransferred to the image carrier while no toner is affixed to the endportions 23 b of the layer-thickness regulating roller 5, i.e., so thata toner layer is not formed on the end portions 23 b. On the other hand,at the central portion 23 a, the potentials are decided in such a mannerthat some of the toner that has attached itself to the central portion23 a of the layer-thickness regulating roller 5 will be left in a thinlayer (one to three layers of toner) on the central portion 4 a of thedeveloping roller 4, while the rest of the toner is removed by causingit to transfer to and be adsorbed by the circumferential surface of thelayer-thickness regulating roller. The excess toner transferred by beingpeeled off by the layer-thickness regulating roller 5 is removed fromthe layer-thickness regulating roller 5 by the resilient removal blade21 disposed in such a manner that its distal end is in abutting contactwith the circumferential surface of the layer-thickness regulatingroller 5. As for the bias potentials, assume that the bias potential ofthe developing roller 4 is about −300 V. If the potential of the centralportion 23 a is made about −150 V and the potential of the end portions23 b is made less than about −50 V, then the effects described above canbe obtained. Of course, the invention is not limited to such potentials,which can be decided freely within limits that provide theabove-described effects.

EXAMPLE 1

A single-layer developing roller 4 having a diameter of 45 mm, ahardness of 35 to 40°, a volume specific resistance of about 3×10⁶ Ω·cmand a surface roughness of about 10 μm was used. The developing roller 4was disposed in such a manner that a development nip width (developmentnip zone) of 4.0 to 7.0 mm was obtained with respect to a drum-shapedOPC (Organic Photo-Conductor) photosensitive drum, and the developingroller 4 was rotated in the forward direction of the image carrier 1 ata peripheral speed substantially identical with that of the imagecarrier 1. A sponge roller having a volume specific resistance of10^(4˜5) Ω·cm was used as the supply roller 6. A bias of −400 V wasapplied to the developing roller 4 and a bias of about −750 V to thesupply roller 6, the developing roller 4 was coated with toner having anaverage particle diameter of 10 μm by the supply roller 6, a toner layerwas then formed by the layer-thickness regulating roller 5 in such amanner that the layer of toner on the developing roller 4 took on athickness that was one to three times the average particle diameter ofthe toner, the toner layer was developed by bringing it into contactwith the image carrier 1 on which had been formed an electrostaticlatent image having a potential of about −750 V in dark areas and apotential of about −80 V is light areas, then the developed image wastransferred to a transfer medium and fixed to obtain an excellent finalimage.

EXAMPLE 2

Use was made of an image carrier 1 comprising a drum-shaped OPCphotosensitive drum having a diameter of 120 mm and a length of about930 mm for supporting size A0, and a developing roller 4 having adiameter of 40 mm, a length of about 930 mm, an apparent surfacehardness of 25 to 40°, a volume specific resistance of about 3×10⁶ Ω·cmand a surface roughness of about 10 μm. The developing roller 4 wasdisposed in such a manner that the depth of bite of the developingroller 4 into the image carrier 1 was about 0.2 to 3 mm ({fraction(1/40)} to {fraction (3/20)} times the radius of the developing roller)and such that the development nip width (development nip zone) was 3.5to 10 mm, and the developing roller 4 was rotated in the forwarddirection of the image carrier 1 at a peripheral speed substantiallyidentical with that of the image carrier 1, namely about 20 mm/s. Asponge roller having a volume specific resistance of 5×10⁴ Ω·cm was usedas the supply roller 6. A bias of −250 to 350 V was applied to thedeveloping roller 4 and a bias of about −350 to 550 V to the supplyroller 6, the developing roller 4 was coated with toner having anaverage particle diameter of 8 μm by the supply roller 6, a toner layerwas then formed by the layer-thickness regulating roller 5 in such amanner that the layer of toner on the developing roller 4 took on athickness that was one to three times the average particle diameter ofthe toner, the toner layer was developed by bringing it into contactwith the image carrier 1 on which had been formed an electrostaticlatent image having a potential of about −550 to 650 V in dark areas anda potential of about 20 V is light areas, then the developed image wastransferred to a transfer medium and fixed to obtain an excellent finalimage.

EXAMPLE 3

A single-layer developing roller 4 having a diameter of 50 mm, ahardness of 40°, a volume specific resistance of about 10⁸ Ω·cm and asurface roughness of about 10 μm was used. The developing roller 4 wasdisposed in such a manner that a development nip width (development nipzone) of 4.8 to 6.0 mm was obtained with respect to a drum-shaped OPCphotosensitive drum, and the developing roller 4 was rotated in theforward direction of the image carrier 1 at a peripheral speedsubstantially identical with that of the image carrier 1. A spongeroller having a volume specific resistance of 10⁵ Ω·cm was used as thesupply roller 6. A bias of −325 V was applied to the developing roller 4and a bias of about −575 V to the supply roller 6, the developing roller4 was coated with toner having an average particle diameter of 8 to 10μm by the supply roller 6, a toner layer was then formed by thelayer-thickness regulating roller 5 in such a manner that the layer oftoner on the developing roller 4 took on a thickness that was one tothree times the average particle diameter of the toner, the toner layerwas developed by bringing it into contact with the image carrier 1 onwhich had been formed an electrostatic latent image having a potentialof about −700 V in dark areas and a potential of about −70 V is lightareas, then the developed image was transferred to a transfer medium andfixed to obtain an excellent final image.

EXAMPLE 4

A single-layer developing roller 4 having a diameter of 100 mm, ahardness of 40°, a volume specific resistance of about 3×10⁷ Ω·cm and asurface roughness of about 10 μm was used. The developing roller 4 wasdisposed in such a manner that a development nip width (development nipzone) of 4.0 to 4.6 mm was obtained with respect to a drum-shaped OPCphotosensitive drum, and the developing roller 4 was rotated in theforward direction of the image carrier 1 at a peripheral speedsubstantially identical with that of the image carrier 1. A spongeroller having a volume specific resistance of 10⁴ Ω·cm was used as thesupply roller 6. A bias of 325 V was applied to the developing roller 4and a bias of about 575 V to the supply roller 6, the developing roller4 was coated with toner having an average particle diameter of 8 μm bythe supply roller 6, a toner layer was then formed by thelayer-thickness regulating roller 5 in such a manner that the layer oftoner on the developing roller 4 took on a thickness that was one tothree times the average particle diameter of the toner, the toner layerwas developed by bringing it into contact with the image carrier 1 onwhich had been formed an electrostatic latent image having a potentialof about 650 to 700 V in dark areas and a potential of about 150 V islight areas, then the developed image was transferred to a transfermedium and fixed to obtain an excellent final image.

EXAMPLE 5

With respect to the image carrier 1 having an electrostatic latent imageof negative polarity, the bias voltage of the developing roller 4 wasmade about −450 V, the bias voltage of the supply roller 6 was madeabout −750 V, the potential of the central portion 5 a oflayer-thickness regulating roller 5 was made about −200 V and thepotential of the end portions 23 b was made less than about −50 V when acopy was being made. During idling, the bias voltage of the developingroller 4 was made about +400 V, the bias voltage of the supply roller 6was made about +750 V, the potential of the central portion 23 a oflayer-thickness regulating roller 5 was made about +650 V and thepotential of the end portions was made about 800 V. It was possible toobtain an effect in which no toner adhered to the end portions 23 b ofthe layer-thickness regulating roller 5 both at copying time and idletime.

EXAMPLE 6

With respect to the image carrier 1 having an electrostatic latent imageof negative polarity, the bias voltage of the developing roller 4 wasmade about −400 V, the bias voltage of the supply roller 6 was madeabout −800 V, the potential of the central portion 23 a oflayer-thickness regulating roller 5 was made about −400 V and thepotential of the end portions 23 b was made about −0 V when a copy wasbeing made. No toner adhered to the end portions 23 b of thelayer-thickness regulating roller 5. Further, during idling, the biasvoltage of the developing roller 4 was made about +350 V, the biasvoltage of the supply roller 6 was made about +750 V, the potential ofthe central portion 23 a of layer-thickness regulating roller 5 was madeabout +350 V and the potential of the end portions was made about 0 V.It was possible to obtain an effect in which no toner adhered to the endportions 23 b of the layer-thickness regulating roller 5.

EXAMPLE 7

With respect to the image carrier 1 having an electrostatic latent imageof negative polarity, the bias voltage of the developing roller 4 wasmade about −250 V, the bias voltage of the supply roller 6 was madeabout −650 V, the potential of the central portion 23 a oflayer-thickness regulating roller 5 was made about −250 V and thepotential of the end portions 23 b was made about −150 V when a copy wasbeing made. No toner adhered to the end portions 23 b of thelayer-thickness regulating roller 5. Further, during idling, the biasvoltage of the developing roller 4 was made about +350 V, the biasvoltage of the supply roller 6 was made about +750 V, the potential ofthe central portion 23 a of layer-thickness regulating roller 5 was madeabout +350 V and the potential of the end portions was made about 0 V.It was possible to obtain an effect in which no toner adhered to the endportions 23 b of the layer-thickness regulating roller 5.

In the development process according to the present invention capable ofbeing realized by the foregoing examples, the traveling speed of thelocal portion of the developing roller 4 that contacts the surface ofthe image carrier 1 in the development nip zone extending from the pointwhere contact with the developing roller 4 starts to the point wherecontact is broken is approximately the same as the peripheral speed ofthe image carrier 1 at the point where contact starts. However, it isbelieved that owing to the resilience of the developing roller 4 per seand due to a change in the radius in the local portion of the developingroller in the development nip zone caused by the developing roller 4biting into the image carrier 1, the developing roller 4 operates so asto break contact with image carrier while the traveling speed of thelocal portion thereof gradually becomes lower than the peripheral speedof the image carrier and thenceforth gradually returns to the originalspeed.

The development nip zone and/or the depth of bite of developing roller 4into image carrier 1 are important factors in order to achieve anoptimum sharp image (and cleaning). Outside of the above-mentioned range(conditions), the contact between the developing roller 4 and imagecarrier 1 is unstable and there is a strong tendency for the appearanceof development unevenness. As a result, it has been discovered that thewidth of the nip zone preferably is 4 mm or greater. Further, it hasbeen clarified that in a case where the radius of the developing roller4 is less than 30 mm, the depth of bite should be {fraction (1/100)} ofthe radius of the developing roller 4 or greater, preferably {fraction(1/40)} of the radius or greater. Furthermore, the motion of the localportion of developing roller 4 in the development nip zone functionswell in regard to cleaning of residual toner on the image carrier 1.Specifically, it is believed that the residual toner on the imagecarrier 1 is subjected to a blade effect for removing the toner by themotion of the local portion that causes a change in traveling speed, asmentioned above, and the roughness on the surface of the developingroller 4, thereby providing excellent cleaning.

Furthermore, in a case where the image carrier 1 and developing roller 4are of the contact type, as illustrated in the above example, there isno limitation upon the toner but, since the toner will leak if theresistance value is too low, a toner having a high resistance orinsulating property of 10⁶ Ω·cm or higher is used. In particular, thetoner desirably is a polymer toner or crushed toner of spherical shapehaving a particle diameter of 5 to 10 μm and an amount of charge of 30μC/g, preferably 50 μC/g or greater.

As shown in FIG. 6, with regard to the traveling speed of the imagecarrier 1 (which comprises a drum-shaped image carrier in theillustrated example), the peripheral speed A thereof and the peripheralspeed B of the developing roller 4 are approximately identical. Thearrangement is such that under these conditions, the developing roller 4contacts the image carrier 1 over a considerable nip zone width and witha considerable depth of bite. As a consequence, the local travelingspeed (the traveling speed of the local portion) of the surface of thedeveloping roller in the zone from the entrance (the side of the contactstarting point) of the development nip zone to the exit thereof (theside of the point where contact is broken) is not uniform. For example,sag at a bulge formed at the entrance causes some delay in speed in theformation of the nip. In addition, the radius of the developing roller 4is shortened owing to gradual squeezing. As a result, the speed (V1) atthe bulge gradually becomes lower than the peripheral speed B of thedeveloping roller 4. This “slowness” brings about maximum slowness on aline connecting the center of the image carrier 1 and the center of thedeveloping roller 4 (owing to maximum shortening of the radius ofdeveloping roller 4). Next, with restoration of the radius toward theexit, the original speed (V2), namely the peripheral speed B, isgradually restored. Strictly speaking, the developing roller 4 subjectsthe surface of the image carrier 1 to a rubbing action at the entranceowing to the aforementioned sag. Similarly, at the exit, the surface ofthe image carrier 1 is pulled owing to the restoration of the developingroller 4 to its original shape by virtue of the resilience of the rollerthat contracted in the nip zone. The developing roller 4 thereforetravels at a speed somewhat higher than the peripheral speed subjectsthe image carrier 1 to a rubbing effect. Owing to these actions, thedevelopment of the electrostatic latent image on the image carrier 1 andthe cleaning of residual toner are carried out effectively.

Thus, in accordance with the present invention, in a contact-typedeveloping apparatus using a non-magnetic single-component toner, asatisfactory width can be set for the development nip zone and asatisfactory depth of bite can be set regardless of any variance in thediameter of the developing roller or any eccentricity of the roller, anychange in diameter caused by a change in environment or any eccentricityof the image carrier. In addition, uneven rotation of the image carrierdoes not occur because development is carried out using approximatelythe same values for the peripheral speed of the image carrier and forperipheral speed of the developing roller. Since the width of thedevelopment nip zone and the depth of bite of the developing roller thatabuts against the image carrier are selected to be large, any changetherein is negligible. Furthermore, the width of the development nipzone (“nip width”) is set to 4 mm or greater, preferably 5 to 10 mm, andthe depth of bite of the developing roller is set to about {fraction(1/100)} M4 of the radius of the developing roller or greater,preferably {fraction (1/40)} or greater. As a result, the peripheralspeed of the toner layer on the developing roller gradually slows down,with respect to the peripheral speed of the surface of the imagecarrier, from the start of contact to substantially the center ofcontact, after which the speed of the toner returns to the peripheralspeed of the image carrier from the center of contact to the point atwhich contact is broken. The developing operation and cleaning operationtherefore take place simultaneously and it is possible to form ajitter-free, extremely sharp image. In particular, it is possible toobtain a sharp image that is free of the occurrence of white stripes,fogging and inadequate density even in development of large-size images.

Further, in a non-magnetic single-component developing apparatus inwhich a developing roller is pressed against an image carrier over apredetermined nip width by contact rollers, drive of the image carrieris transmitted to the developing roller by gears, thereby eliminatingblurring of the image and making it possible to obtain stable images.Further, when the developing apparatus is not being used, the developingapparatus is moved in a direction away from the image carrier and thegear on the image carrier is meshed with the gear on the developingroller slightly to such an extent that the gears do not separatecompletely. As a result, the gears can mesh smoothly and will not bedamaged by re-application of the pressing force from the developingapparatus.

Furthermore, owing to a simple arrangement in which a layer-thicknessregulating roller is utilized above the developing roller, affixing oftoner to both end portions of the developing roller is prevented, as aresult of which it is possible to effectively prevent leakage of tonerfrom both ends of the developing roller to the exterior of thedeveloping apparatus.

What is claimed is:
 1. A developing apparatus for forming a thin layerof toner, comprising: a resilient developing roller for receiving anon-magnetic single-component toner; means for bringing said roller intoabutting contact with a surface of an image carrier, whereby toner onthe resilient developing roller is supplied to an electrostatic latentimage previously formed on the surface of the image carrier, therebydeveloping the electrostatic latent image; and means for moving theimage carrier and the resilient developing roller in a forward directionso that a travelling speed of the image carrier and a peripheral speedof the resilient developing roller become substantially identical,wherein: a width of a development nip zone that extends from a point atwhich the resilient developing roller starts to contact the imagecarrier to a point at which the resilient developing roller breakscontact with the image carrier is at least 4 mm, a rubber hardness ofthe resilient roller is 20 to 40° measured according to the JIS K 6253(Type A) standard and a diameter of the resilient developing roller iswithin the range 40 to 100 mm, and an amount of bite by which thesurface of the image carrier bites into the resilient developing rolleris equal to or greater than {fraction (1/100)} of the radius of theresilient developing roller, and the amount of bite and the width of thedevelopment nip zone are such that the resilient developing rollerbreaks contact with the image carrier while a travellng speed of a localportion of the resilient developing roller in contact with the surfaceof the image carrier gradually decreases from the point at which contactstarts and thenceforth gradually returns to the original speed due toresilience of the resilient developing roller.
 2. The developingapparatus according to claim 1, wherein said amount of bite is at least{fraction (1/40)} of the radius of the developing roller.
 3. Thedeveloping apparatus according to claim 2, wherein said amount of biteis 0.2 to 3 mm.
 4. The developing apparatus according to claim 3,wherein the thin layer of toner formed on said resilient developingroller is a uniform layer of one to three layers of toner.
 5. Thedeveloping apparatus according to claim 4, additionally comprising meansfor forming said thin toner layer, which means comprises alayer-thickness regulating roller located in opposition to thedeveloping roller.
 6. The developing apparatus according to claim 5,wherein said layer-thickness regulating roller has a central portionalong the axial direction thereof and end portions that are electricallyinsulated from the central portion, and the developing apparatus furthercomprises means for applying a bias voltage to the end portions forpreventing adhesion of toner to the end portions of the resilientdeveloping roller.
 7. The developing apparatus according to claim 6,further comprising a scraping blade located in pressured contact with acentral portion of said layer-thickness regulating roller with respectto the axial direction thereof and with end portions of said roller,said scraping blade adapted for scraping off toner that has adhered tothe layer-thickness regulating roller.
 8. The developing apparatusaccording to claim 1, wherein said developing roller is configured forcontacting and separating from the surface of said image carrier, bothends of said developing roller comprise contact rollers, and saidcontact rollers are in abutting contact with both ends of said imagecarrier to regulate the width of said development nip zone.
 9. Thedeveloping apparatus according to claim 1, further comprising means formoving said image carrier and said developing roller to mesh a gearlocated on a flange of the image carrier with a gear located on an endof the developing roller, whereby drive force from said image carrier istransmitted to the developing apparatus.
 10. The developing apparatusaccording to claim 9, further comprising separation means for causingsaid developing apparatus to separate from said image carrier, whereinthe gear located on said image carrier and the gear located on saiddeveloping roller are slightly meshed when said image carrier and saiddeveloping roller are in a separated state.
 11. The developing apparatusaccording to claim 10, further comprising a cam in abutting contact witha portion of the developing apparatus for being turned at introductionof power to thereby move the developing apparatus in such a manner thatsaid developing roller is pressed against said image carrier, and byprovision of a capacitor charged when power is being introduced, whereinsaid capacitor is switched over to act as a power source at cut-off ofpower, thereby rotating said cam and moving the developing apparatus insuch a manner that said developing roller moves in a direction in whichit separates from said image carrier.
 12. The developing apparatusaccording to claim 1, wherein the thin layer of toner formed on saidresilient developing roller is a uniform layer of one to three layers oftoner.
 13. The developing apparatus according to claim 1, wherein saidmeans for forming said thin toner layer comprises a layer-thicknessregulating roller placed in opposition to the developing roller.
 14. Thedeveloping apparatus according to claim 13, wherein said layer-thicknessregulating roller has a central portion along the axial directionthereof and end portions that are electrically insulated from thecentral portion, and further comprises means for applying a bias voltageto the end portions for preventing adhesion of toner to the end portionsof the resilient developing roller.
 15. The developing apparatusaccording to claim 14, further comprising a scraping blade in pressuredcontact with a central portion of said layer-thickness regulating rollerwith respect to the axial direction thereof and with end portions ofsaid roller, said scraping blade adapted for scraping off toner that hasadhered to the layer-thickness regulating roller.
 16. A developingapparatus for forming a thin layer of toner, comprising: a resilientdeveloping roller for receiving a non-magnetic single-component toner;and means for bringing said roller into abutting contact with a surfaceof a drum-shaped image carrier, whereby toner on the resilientdeveloping roller is supplied to an electrostatic latent image thatpreviously formed on the surface of the image carrier, therebydeveloping the electrostatic latent image, wherein: moving the imagecarrier and the resilient developing roller in a forward direction sothat a travelling speed of the image carrier and a peripheral speed ofthe resilient developing roller become substantially identical, andsetting an amount of bite by which the surface of the image carrierbites into the resilient developing roller and width of a developmentnip zone that extends from a point at which the resilient developingroller starts to contact the image carrier to a point at which theresilient developing roller breaks contact with the image carrier, sothat the resilient developing roller breaks contact with the imagecarrier while a travelling speed of a local portion of the resilientdeveloping roller in contact with the surface of the image carriergradually decreases from the point at which contact starts andthenceforth gradually returns to the original speed owing to resilienceof the resilient developing roller, a layer-thickness regulating rollerlocated in opposition to the developing roller and serving as means forforming said thin toner layer has a central portion along the axialdirection thereof and end portions that are electrically insulated fromthe central portion, and a bias voltage for preventing adhesion of tonerto the end portions of the resilient developing roller is applied to theend portions.
 17. The developing apparatus according to claim 16,further comprising a scraping blade located in pressured contact with acentral portion of said layer-thickness regulating roller with respectto the axial direction thereof and with end portions of said roller,said scraping blade adapted for scraping off toner that has adhered tothe layer-thickness regulating roller.
 18. A developing apparatus forforming a thin layer of toner, comprising: a resilient developing rollerfor receiving a non-magnetic single-component toner; and means forbringing said roller into abutting contact with the surface of an imagecarrier, whereby toner on the resilient developing roller is supplied toan electrostatic latent image previously formed on the surface of theimage carrier, thereby developing the electrostatic latent image,wherein: moving the image carrier and the resilient developing roller ina forward direction in such a manner that a travelling speed of theimage carrier and a peripheral speed of the resilient developing rollerbecome substantially identical, setting an amount of bite by which thesurface of the image carrier bites into the resilient developing rollerand width of a development nip zone that extends from a point at whichthe resilient developing roller starts to contact the image carrier to apoint at which the resilient developing roller breaks contact with theimage carrier so that the resilient developing roller breaks contactwith the image carrier while travelling speed of a local portion of theresilient developing roller in contact with the surface of the imagecarrier gradually decreases from the point at which contact starts andthenceforth gradually returns to the original speed owing to resilienceof the resilient developing roller, moving said image carrier and saiddeveloping roller to mesh a gear located on a flange of the imagecarrier with a gear located on an end of the developing roller, wherebydrive force from said image carrier is transmitted to the developingapparatus; and by having separation means for causing said developingapparatus to separate from said image carrier, wherein the gear locatedon said image carrier and the gear located on said developing roller areslightly meshed when said image carrier and said developing roller arein a separated state.
 19. The developing apparatus according to claim18, further comprising a cam in abutting contact with a portion of thedeveloping apparatus for being turned at introduction of power tothereby move the developing apparatus in such a manner that saiddeveloping roller is pressed against said image carrier, and byprovision of a capacitor charged when power is being introduced, whereinsaid capacitor is switched over to act as a power source at cut-off ofpower, thereby rotating said cam and moving the developing apparatus insuch a manner that said developing roller moves in a direction in whichit separates from said image carrier.
 20. The developing apparatusaccording to claim 1, wherein said width of a development nip zone is 5to 10 mm.
 21. A developing apparatus for forming a thin layer of toner,comprising: a resilient developing roller for receiving a non-magneticsingle-component toner; and means for bringing said roller into abuttingcontact with a surface of a drum-shaped image carrier, whereby toner onthe resilient developing roller is supplied to an electrostatic latentimage that previously formed on the surface of the image carrier,thereby developing the electrostatic latent image, wherein: moving theimage carrier and the resilient developing roller in a forward directionso that a travelling speed of the image carrier and a peripheral speedof the resilient developing roller become substantially identical, andsetting an amount of bite by which the surface of the image carrierbites into the resilient developing roller and width of a developmentnip zone that extends from a point at which the resilient developingroller starts to contact the image carrier to a point at which theresilient developing roller breaks contact with the image carrier, sothat the resilient developing roller breaks contact with the imagecarrier while a travelling speed of a local portion of the resilientdeveloping roller in contact with the surface of the image carriergradually decreases from the point at which contact starts andthenceforth gradually returns to the original speed owing to resilienceof the resilient developing roller, the size of an image represented bysaid latent image is at least A2 size.